The cheeky Union Jack boot is the first to make it down the runway in the show’s finale. Costume designer Gregg Barnes calls it an “irreverent” nod to the flag. Photo: Matthew Murphy

Midway through Harvey Fierstein’s new Broadway sensation, “Kinky Boots,” the entire cast breaks out into a splashy musical number that causes the audience to gasp and erupt into applause.

The brilliant star of the performance, now in previews, that has them so rapt? That would be an over-the-knee, lace-up, 6-inch-heeled boot made from three types of red leather, which arrives onstage via conveyor belt.

“Kinky Boots,” with music by Cyndi Lauper, tells the story of a young Englishman who, in a bid to save his late father’s shoe factory, converts the stodgy manufacturer into a purveyor of the finest boots for drag queens.

And the show’s fancy footwear is causing major buzz on the Great White Way.

Audience member Matt Vanderberg, who flew in from San Francisco to celebrate his 40th birthday at the Al Hirschfeld Theatre on Saturday, went so far as to suggest that they be recognized at the Tony awards.

“The boots should have their own category!” crows Vanderberg. “And the Tony for the best accessory in a Broadway show goes to . . . ‘Kinky Boots!’

“Show me where to buy them!”

Another attendee, Lisa Negrete of Chicago, wonders the same thing: “I want to try them on!” exclaims the 45-year-old.

Unfortunately for smitten showgoers, the pairs of boots on display — including one emblazoned with the Union Jack and another inspired by the British Beefeaters — cost up to a whopping $2,500 each, and aren’t available for public consumption.

“People go up to the lobby and want to buy a pair because they don’t realize that we make them [for the show],” says costume designer Gregg Barnes, who defines a “kinky boot” as “a shoe that lets you live out your fantasy. It has a sexy, cheeky quality.”

But he was presented with a unique set of challenges when it came to their construction — after all, these boots weren’t destined to be worn by dainty female hoofers, but rather muscular male dancers performing overly physical and sometimes acrobatic choreography.

“I’m not a shoe designer,” says Barnes. “I don’t wear high heels.”

That’s where local cobblers T.O. Dey and Phil LaDuca (a former Broadway dancer) came in. LaDuca — who has a reputation for making shoes beautiful enough for their close-ups and sturdy enough to support a dancer’s rigorous regime — made Barnes’ sketches come to life.

The boots were in gestation since last March, and took a month to manufacture both here and in Puglia, Italy.

“What we did exactly parallels the story,” says Barnes. “We had to make them strong enough so [men] could dance in them. We did so much research and development.”

The costume designer suffered a setback when two of the heels broke early in the show’s Chicago run in October. “We said, ‘Oh, back to the drawing board.’ ”

Barnes worked through the kinks by reinforcing certain parts of the shoes according to each dancer’s physical needs.

“We haven’t had any breaks since,” says Barnes.

But this doesn’t mean the sky-high heels are any easier to wear, as the designer, who’s tried on his towering boots, can attest.

“To be honest, I couldn’t get them off quick enough,” says Barnes. “It was uncomfortable but empowering. I have to give a shout-out to the cast. A lot of them are putting on high heels for the first time, and you use different muscles. We never had a single person say, ‘I can’t.’ ”

The whole production, which opens on April 4, culminates in a jaw-dropping fashion show that will be sure to delight even the casual fan of shoe porn.

“My heart stopped when they did this whole number building up to the entrance of my boots,” says LaDuca. “They’re my proudest moment as a shoe designer.”

And while the boots onstage aren’t available for purchase, Barnes says this hasn’t stopped audience members from showing up in their own kinky soles.

“There was a guy the other night who had on skinny jeans, a sports coat and leopard high heels,” says Barnes, adding with a giggle, “It’s like when you’re a kid and you go to ‘Princesses on Ice’ with a princess costume!”